The present invention relates to instrumentation for measuring blood flow in vessels of the retina by Doppler velocimetry.
The general theory of laser Doppler velocimetry, as applied to the measurement of a flowing fluid such as blood inside a blood vessel, is a well known application of flow measurement technology. Briefly, monochromatic light aimed at the vessel and into the flowing blood is reflected by the blood cells as diffuse light with a frequency distribution corresponding to the components of velocity of the individual scatterers. By analyzing the frequency distribution of the reflected light at two fixed receivers with a known separation angle, the velocity or, ideally, the velocity profile of the flowing blood can be deduced.
When one attempts to apply this approach to detect blood flow rates in vessels of the retina, however, practical obstacles are encountered. First, individual retinal vessels have a diameter under several hundred microns, so that in order to perform a reliable measurement it is necessary to aim a beam of laser light of diameter approximately equal to the diameter of the vessel. Smaller beam diameters introduce the risk of missing the centerline flow measurement, while larger beam diameters result in a lower signal to background ratio.
Second, the Doppler analysis requires collection of the reflected light from two distinct directions having a specified angular separation. This light collection must be done outside the eye. The optical paths therefore will vary depending on the curvatures of the eye involved, and the collected light will include extraneous light due to reflection at various surfaces of the eye.
Third, it is necessary to perform this aiming and to collect a sufficiently strong return signal, despite relatively fast and large scale movements of the eyeball.
When it is considered that a small diameter beam must be used to maintain an acceptable signal to noise ratio, and that the level of reflected light from the fundus that can be collected outside the eye is highly attenuated, the foregoing obstacles are seen to impose severe limits on the quality of collected light available for Doppler analysis.
These difficulties have heretofore limited the clinical applicability of laser Doppler velocimetry to carefully controlled and rather cumbersome analytical investigations. Typically, the procedure is done by fitting a rectifying lens directly on the cornea, and then, with the illumination and collection optics manually positioned on a target vessel, recording short time segments of the collected spectra. A large number of such recordings are then analyzed and segments are pieced together to obtain an analytically derived synthetic recording representing the flow during one or more entire heartbeat intervals. The analysis and ultimate synthesis or identification of a representative one- or two-second Doppler spectrum is done some time after the recording, so that blood flow information is not quickly provided.
One approach to simplifying the processing of the recorded Doppler spectra is to develop algorithms for initially selecting only those recorded spectrum segments which meet certain criteria representative of the expected flow functions. Highly noisy or anomalous recording segments are discarded, thus limiting the amount of remaining data that must be processed. This approach, while clearly eliminating records resulting, for example, when the beam misses a vessel entirely, may screen out some valid flow information and render the system blind to clinically significant details. Analysis of Doppler records would be simplified if the instrumentation could be aimed with sufficient stability to record a continuous record having a duration of a full heartbeat interval or longer. More meaningful measurements of blood flow could also be obtained if the stability were sufficient to allow aiming a Doppler illumination spot on a central region of a blood vessel and on smaller vessels.